This document relates to a magnetically latching solenoid, and more particularly, to a position sensor for detecting the position of a plunger in a magnetically latching solenoid.
A magnetically latching solenoid has an advantage over conventional solenoids in that no control power is required to maintain a plunger of a magnetically latching solenoid in either of two possible stable positions. Magnetically latching solenoids are described in detail in U.S. Pat. No. 3,022,450 to Chase. By contrast, the plunger of a conventional non-latching solenoid is held by a spring in a first position when no current is applied to coil in the solenoid, and is driven to a second position by magnetic forces whenever sufficient current is applied to the coil. Such current must be continuously maintained as long as it is desired for the solenoid plunger to occupy the second position.
FIGS. 1-3 illustrate a magnetically latching solenoid, such as those described in the U.S. Pat. No. 3,022,450. It should be noted that the solenoid illustrated in FIGS. 1-3 has a box frame with open sides rather than the closed tubular frame shown in the U.S. Pat. No. 3,022,450. It should be noted that the position sensor disclosed in this document will work with either type of frame.
In FIG. 1, magnetically latching solenoid 100 may include a nonmagnetic shaft 102 that projects out of the frame 104 at one or both of opposing sides 105A and 105B. One purpose of the shaft 102 may be to guide and support the internal moving components of the solenoid 100. The shaft 102 also can be attached to external components (not shown), so that the external components can be caused to move by the solenoid 100. Solenoid 100 also includes two coils 106 inside the frame 104, and a permanent magnet structure including two magnets 108 between the coils 106.
FIG. 2A shows solenoid 100 with both coils 106 made invisible for added clarity. With coils 106 invisible, additional components of solenoid 100 may be seen. Cylindrical steel anvils 110A and 110B are attached to the inside surface of the left and right end of the frame 104. In this example, anvil 110A is attached to the left side 105A of solenoid 100 and anvil 110B is attached to the right sides 105B. Between the anvils 110A and 110B is a cylindrical steel plunger 112, which may be attached to the shaft 102 so that the plunger and the shaft can both slide left or right together, thus moving the shaft through an opening in one or both anvils until the plunger strikes one of the anvils. This motion of the plunger 112 and of the shaft 102 together is called the stroke of the solenoid 100. In FIG. 2 the plunger 112 and the shaft 102 are at the left end of their stroke.
With coils 106 made invisible, FIG. 2A also shows that the outer surfaces of the magnets 108 make contact with the inside surfaces of the top and the bottom of the frame 104, and that the inner surfaces of the permanent magnets make contact with coupler 114. The coupler 114 may be made from a magnetic material (such as steel) and has a large hole through which the plunger 112 passes, with a small clearance so that the coupler does not touch the plunger. One purpose of the coupler 114 is to conduct the magnetic flux from the magnets 108 into the plunger 112, thereby facilitating the latching of the plunger 112, and thus, the shaft 102, at either end of its stroke. An alternative construction may avoid the coupler 114 by vertically extending the magnets 108 toward a center-plane of the solenoid 100. Semicircular notches may be added to the extended magnets 108 such that the magnets deliver any magnetic flux directly to the plunger 112 across a small air gap.
Similar to FIG. 2A, FIG. 2B shows solenoid 100 with frame 104 and coils 106 made invisible.
FIG. 3 shows the same components as FIG. 2B, but with the anvils 110A and 110B made invisible, and viewed from the right end. FIG. 3 also shows that both magnets 108 are oriented so that their north poles are in contact with the coupler 114, and their south poles are in contact with the frame 104 (not shown in FIG. 3). The solenoid 100 would also work as well if the poles of both magnets 108 were reversed. The arrows represent the magnetic flux inside the magnets 108, the steel coupler 114, or the steel plunger 112. In this example, both magnets 108 drive magnetic flux into the coupler 114, from which the magnetic flux crosses the clearance gap 120 into the plunger 112. In an alternative construction with notched magnets such as the embodiment illustrated in FIG. 3A, the magnets 108 may direct magnetic flux directly across the gap 120 into plunger 112.
A magnetically latching solenoid latches because most of the magnetic flux tends to follow the path of least reluctance, which is the path that includes the largest portion in a high permeability material such as steel, and the least portion in air. When the plunger is at or near one end of its stroke, most of the flux from the magnets tends to pass through the shorter air gap, with very little passing through the longer air gap at the other end of the plunger.
The attractive forces produced on the flat ends of the plunger 112 are proportional to the square of the magnetic flux density there. Therefore, the attractive force across the shorter air gap will be much greater than the attractive force across the longer air gap. The difference between these forces will tend to hold or latch the plunger at the end of its stroke, without any current in the coil or coils.
A magnetically latching solenoid may be caused to change position by energizing one or both coils with a polarity such that the flux from the coil surrounding the shorter air gap tends to oppose the flux created in the shorter air gap by the magnets. When the attractive force in the shorter air gap becomes weak enough, the attractive force in the longer air gap may overcome it and cause the plunger to move. Once the plunger nears the opposite end of its stroke, the opposite air gap will become the shorter one, and the solenoid will latch in its new position.
The magnets 108 have a characteristic maximum flux density, which depends on the material from which the magnets are made. For example, Neodymium-Iron-Boron magnets have a maximum flux density of about 1.2 Tesla. By comparison, steel is capable of conducting a flux density of about 2.0 Tesla before it saturates.
To obtain large latching forces it is desirable to maximize the flux density at the flat ends of the plunger 112. The magnets 108 may be chosen to have a cross-sectional area larger than the cross-sectional area of the plunger 112. When the coupler 114 conducts the magnetic flux from the magnets 108 into the plunger 112, the magnetic flux is concentrated into a smaller cross-sectional area, and the flux density in the plunger is thereby increased over the flux density in the magnets, thereby increasing the latching forces that may be produced on the plunger. The maximum possible latching forces may be achieved when the plunger 112 reaches a flux density where its steel is saturated.
With a conventional solenoid it is possible to deduce the position of the plunger of the solenoid by detecting the presence or absence of sufficient current in the solenoid coil. This method is not feasible with a magnetically latching solenoid because the plunger may occupy either position when the coils are not energized. Therefore it is generally necessary to add extra components to a magnetically latching solenoid for the purpose of detecting the plunger position. Such extra components could include a micro-switch mounted on the stationary portion of the solenoid, with an actuator mounted on the moving portion of the solenoid. Depending on the position of the plunger, and thus the actuator, the switch would indicate whether the plunger is in a first or second position. Other possible extra components could include an optical sensor or a magnetic proximity sensor, but all share the drawback that an extra moving component is required, which decreases the reliability of the solenoid. For the case of a micro-switch, reliability is further decreased because the electrical contacts inside the micro-switch may become contaminated or corroded.